chromeos: Stop creating symlinks in DriveCacheTest
DriveCache will stop creating symlinks, tests should not rely on symlinks.
TestInitializeCache is removed because this test does not add any test coverage.
BUG=159760
TEST=unit_tests --gtest_filter="DriveCacheTest.*"
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=167625